In a nuclear reactor, the pressure tubes which contain the fuel bundles are each positioned within a calandria tube. It is necessary to have an annular space maintained between the pressure tube and the calandria tube to allow for the circulation of gases which thermally insulate the hot pressure tube from the relatively colder calandria tube and the heavy water moderator which flows in the space outside the calandria tube.
Conventionally, a garter spring spacer has been used to maintain the space between the pressure tube and the calandria tube. A garter spring spacer is an endless helical spring disposed around the pressure tube. Its convolutions contact the walls of both the pressure tube and the calandria tube. The spring is unattached to either tube. A garter spring spacer was disclosed in U.S. Pat. No. 3,106,520 issued to Wolfe et al. Oct. 8, 1963.
While the garter spring is effective to support the pressure tube within the calandria tube, there are a number of drawbacks inherent in its use. Because garter spring spacers are not attached to either the pressure tube or the calandria tube, they must be installed on the pressure tube after the pressure tube has been placed inside the calandria tube. As a result, installation of the garter spring spacers is a difficult procedure which requires tedious operations to be carried out at the reactor face. The problem is exacerbated over the operating time of the fuel channel as increased sag develops in the calandria tubes.
The difficulty installing the spacers is of particular significance to the fuel channel replacement procedures because each time the fuel channels are replaced, the spacers must be re-installed. Consequently, the procedure is slow and expensive. An improved fuel channel replacement procedure is desirable not only to reduce the time and expense of the operation but also to reduce the radiation dose level to which those who replace the fuel channels may be exposed.
Another problem with garter spring spacers is that they have a tendency to move axially along the pressure tube during operation of the reactor. This movement is due to vibration and thermal cycling. When axial movement occurs, the spacers must be repositioned. Repositioning the spacers is difficult and costly and may also result in radiation exposure to those who conduct the procedure.
Furthermore, the geometry of the garter spring spacer has limited the materials which can be used in its manufacture. Garter spring spacers have conventionally contained Inconel. It would be desirable to use only zirconium alloy to reduce fuel burn-up and increase neutron efficiency, however the brittleness of that material renders it difficult to form into a spring. Further, there would be considerable concern that a garter spring formed only of zirconium alloy would break during the life cycle of the reactor.
Garter spring spacers are also known to cause wear on the inner wall of the calandria tube which may reduce the burst strength of the tube.
Thus there is a need for a pressure tube spacer which is effective to maintain a space between the pressure tube and the calandria tube and which overcomes some of the difficulties inherent in the use of prior art spacers such as the garter spring spacer.